1. Field of the Invention
The invention pertains to the field of high voltage switches. More particularly, the invention pertains to solid state high voltage switches used to discharge energy storage capacitors in modulators, pulsed lasers, particle accelerators, radars and other applications.
2. Description of Related Art
High voltage switches capable of operation at high peak currents with high rate of current risetime for short pulses are used in modulators for high power RF and microwave amplifiers used in particle accelerators, radar and defense applications. Other applications requiring these switches include medium and high power pulsed lasers, kicker magnet drivers, compact x-ray sources for medical and industrial applications, pulsed electron, ion or radiation sources for material processing and food and water purification.
Typically, thyratrons switch tens of kilovolts at high current and at high rate of current risetime for many of these applications. Ignitrons and spark gaps switches are also employed. All of these devices have one or more of the following disadvantages: high cost, limited lifetime, and expensive maintenance and/or replacement. In addition, warm-up periods are often required, and the devices may exhibit performance changes over time. Finally, the mounting, cooling and triggering of these devices can be complex.
Solid state devices, such as thyristors, can have long lifetimes and minimal maintenance costs, but are typically capable of operation at only a few kilovolts and at maximum current risetimes of a few kiloamps per microsecond. Thyristors can be connected in series for higher voltage operation, but fast rising high current gate pulses for each thyristor are required to insure near-simultaneous turn-on and kA/xcexcs switch current risetimes. Because each thyristor in the stack is floating at a different potential, the gate current pulses must be isolated. Pulse transformers at each gate, with a single high current trigger generator driving the transformer primaries, can accomplish this isolation. This approach, which includes the requirement for a saturable inductor, discussed in the next paragraph, is similar to the switch described in U.S. Pat. No. 5,331,234. However, pulse transformers capable of tens of kV""s isolation with the low leakage inductance necessary to maintain the fast rising high current gate pulse, are large and costly. An alternate approach is to use isolated gate drive circuits for each device, triggered by low current pulses delivered via much smaller pulse transformers. However, the DC power source for each gate drive must be isolated.
The switch described in U.S. Pat. No. 5,331,234 uses energy in the snubber circuit to help turn on devices (but not to trigger them). This energy would not be there in the event of a load fault.
The individually packaged thyristors must be stacked, clamped, cooled, and triggered with appropriate insulation. Also, for high di/dt applications, saturable inductors, (also referred to as magnetic assist or magnetic switches) are typically employed to allow sufficient time for complete turn-on of the thyristor before it can conduct large currents. Generally, it is necessary to design the saturable inductor for each specific application. These criteria make for a switch that is bulky, expensive, and complex to install, mount, trigger and cool. In addition, the user is often responsible for designing, assembling and testing the switch.
The size and trigger system requirements preclude the use of these thyristor stacks as drop-in replacements for thyratrons in existing systems.
Insulated gate bipolar transistors (IGBT) can also be used to create a high voltage, high power switch. IGBT""s are generally capable of a shorter turn-on time and a faster risetime than thyristors, and are much easier to turn off. However, IGBTs have higher on-state loses than thyristors. (The conduction physics for an IGBT is inherently more lossy than the conduction physics for thyristors.) Large arrays of IGBT""s connected in series and parallel are necessary to achieve the voltage and current requirements. More complex drive circuitry is also required. As a consequence, IGBT switches with the same voltage and power handling capabilities as thyristor switches are larger and more expensive. Many applications do not require the turn off capability of the IGBT.
In order to trigger any thyristor for a high current, high di/dt application, particularly if multiple devices are used in series or parallel, it is necessary to drive the thyristor gate with a large, fast rising current pulse. For all configurations where thyristors are connected in series and many applications in which only a single device is used, there is a large DC potential difference between the gate of the thyristor and the trigger signal, and some form of DC isolation must be provided. A high power trigger transformer capable of high voltage isolation rapidly increases in bulk and cost as the voltage, current and di/dt requirements of the thyristor increase.
This invention is a high voltage switch capable of high conduction current with a high rate of current risetime (high di/dt). The switch has a long lifetime, and is rugged, reliable, economical, compact and easy to trigger. The small size and freedom from auxiliary systems will enable the switch to be used as a direct replacement for thyratrons and other high power switches in existing and new systems. These solid-state switches will be more reliable and have a much longer lifetime than the switches they replace.
The high voltage switch described in this invention can be used in applications requiring the rapid discharge of pulse forming networks, capacitors or other energy storing elements. Applications include pulsed lasers, modulators used in particle accelerators, radar systems, industrial processing and medical imaging, and pulsed electron, ion or radiation sources for material processing and food and water purification.
The invention uses a low voltage, low current trigger that can be easily isolated by a small inexpensive transformer to trigger a gate drive circuit floating at the same potential as the thyristor gate. The gate drive circuit obtains its energy from the energy that is being switched in the main circuit. The gate drive circuit can also be triggered with an optical signal, eliminating the trigger transformer altogether. This approach makes it easier to connect many thyristors in series to obtain the desired hold off voltage. Each thyristor has its own gate drive circuit that floats at gate potential.